US President Donald Trump has called on European allies to take back more than 800 Islamic State fighters captured in Syria – and put them on trial.

President Trump said "the alternative is not a good one in that we will be forced to release them," with the terror group's caliphate in Iraq and Syria "ready to fall."

He tweeted: "The US does not want to watch as these ISIS fighters permeate Europe, which is where they are expected to go.

"We do so much, and spend so much - Time for others to step up and do the job that they are so capable of doing.

He claimed the US was pulling out of Syria, “after 100% caliphate victory."

President Trump was tweeting after US Vice President Mike Pence said US-backed forces were fighting for the last mile of IS-controlled territory in Syria.

Ciya Furat, a commander with the Kurdish-led Syrian Democratic Forces (SDF), told reporters on Saturday that Islamic State fighters were contained to an area of 700 square metres in the village of Baghuz near the Iraqi border.

A US-backed Syrian Democratic Forces (SDF) fighter stands atop a hill in the desert outside the village of Baghouz, Syria, 14-02-2019. Image: AP Photo/Felipe Dana

Thousands of civilians have been streaming out of Baghuz this week to escape the violence.

The US withdrawal from Syria has sparked fears that the terror group will simply return to prominence as soon as the soldiers are gone.

Activists and residents have warned that IS still has sleeper cells in Syria and Iraq and is laying the groundwork for further insurgency.

At the height of its power in 2014, IS controlled nearly a third of Iraq and Syria.

Additional reporting from IRN